1. Claim of Priority
This application claims priority to Korean Patent Application No. 10-2009-0104233 filed on Oct. 30, 2009.
2. Field of the Invention
The present invention relates to a DC offset suppression circuit for a complex filter used in the Low-IF communication circuit, and more particularly, to a DC offset suppression circuit which utilizes a phase compensation circuit with resistance ratios. The phase compensation circuit suppresses the DC offset of the complex band-pass filter by feeding back the compensated phase to an input of the complex filter through the DC feedback unit.
3. Description of Prior Art
Under ordinary circumstances of wireless communication systems, signal received by an antenna is amplified by a low noise amplifier (LNA) and mixed with carrier signals generated by a frequency generator in a mixer. Then, the signals are input to a filter to detect reception signals. At this time, the signals received by the antenna and the frequency of the carrier signals are equalized in the mixer. There are two ways to transmit the signals. One way is called a direct conversion (DC) where the signals are transmitted to a DC frequency, and the other way is called a low-intermediate-frequency (low-IF) where the signals are transmitted close to an approximate DC intermediate frequency.
However, carrier signals generated by the frequency generator in the mixer may perform self-mixing when the antennae do not receive signals, or in other words, when the LNA does not output any signals.
If the mixer performs self-mixing as mentioned above, a filter, a programmable gain amplifier (PGA), or variable gain amplifier (VGA) at the posterior end of the mixer will generate self-gain, which causes to amplify DC components. The amplified DC components may cause damage in transistors inside the filter or PGA. In order to solve this problem, a DC feedback circuit is usually utilized to suppress the amplified DC offset due to the self-gain of the filter or PGA.
Referring to FIG. 1 and FIG. 2, FIG. 1 illustrates a block diagram of a receiver of conventional communication systems, and FIG. 2 illustrates a circuit diagram of a filter of a conventional DC offset suppression circuit.
As FIG. 1 and FIG. 2 show, a radio frequency (RF) receiver comprises a low noise amplifier (LNA) 1, a mixer 2, a filter unit 3, and a programmable gain amplifier (PGA) 4. The LNA 1 receives RF signals from antennae. The mixer 2 mixes output signals from the LNA 1 and intermediate frequency signals. The filter unit 3 filters output signals from the mixer 2. The PGA 4 amplifies output signals from the filter unit 3, which comprises a filter A0 and a DC feedback unit B0. The DC feedback unit B0 is used to feed back the DC to suppress the DC offset. An intermediate frequency generating unit 5, comprising a voltage-controlled oscillator (VCO) and a phase lock loop (PLL), generates intermediate frequency signals. The mixer 2 is used to mix the intermediate frequency signals generated from the intermediate frequency occurring unit 5 and the output signals from the LNA 1.
As mentioned above, the DC offset suppression method adopted by the direct-conversion communications adopts is that, the entire high-pass filter (HPF) A0 and the DC feedback unit B0 form a single feedback loop to suppress the DC offset.
However, under circumstances where a complex filter is used, a phase error will occur when signals pass through the above-mentioned DC feedback unit to suppress the DC offset.
Referring to FIG. 3, FIG. 3 is a circuit diagram illustrating a traditional complex filter. As FIG. 3 shows, the traditional complex filter comprises a first filter unit 31, a second filter unit 32, and a frequency-changing unit 33. The first filter unit 31 filters a first input Viinm and a second input Viinp output by a mixing circuit and generates a first output Vioutp and a second output Vioutm. The second filter unit 32 filters a third input Vqinm and a fourth input Vqinp output by the mixing circuit and generates a third output Vqoutp and a fourth output Vqoutm. The frequency-changing unit 33 feeds back the outputs from the first filter unit 31 to an input of the second filter unit 32 and then feeds back the outputs from the second filter unit 32 to an input of the first filter unit 31. That is, the first filter unit 31 filters I signals; the second filter unit 32 filters Q signals; the frequency-changing unit 33 feeds back the I signals to an input of the Q signals and then feeds back the Q signals to an input of the I signals by means of resistors. In this way, the frequencies are altered.
Therein, there is a 180-degree phase difference between the first input Viinm and the second input Viinp. The phase of the third input Vqinm is 90 degrees relative to the phase of the first input Viinm, and the phase of the fourth input Vqinp is 90 degrees relative to the phase of the second input Viinp. Similarly, there is a 180-degree phase difference between the first output Vioutp and the second output Vioutm. The phase of the third output Vqoutp is 90 degrees relative to the phase of the first output Vioutp, and the phase of the fourth output Vqoutm is 90 degrees relative to the phase of the second output Vioutm.
As shown in FIG. 4, the outputs are fed back to the inputs alternatively via the frequency-changing unit 33 of the complex filter and the frequencies are altered by adjusting the resistance of the frequency-changing unit 33.
Therefore, when self-mixing occurs in the complex filter where only a traditional DC offset suppression circuit, that is, an ordinary DC feedback unit, is adopted, phase will vary due to the characteristics of the complex filter.
Referring to FIGS. 5a and 5b, FIG. 5a is an equivalent-circuit diagram showing an ordinary low pass filter, and FIG. 5b is an equivalent-circuit diagram showing an ordinary complex filter. A formula for the low pass filter shown in FIG. 5a is as follows:
                              Vout          Vin                =                                            R              ⁢                                                          ⁢                              2                /                R                            ⁢                                                          ⁢              1                                      1              +                              sR                ⁢                                                                  ⁢                2                ⁢                C                ⁢                                                                  ⁢                2                                              =                                                    R                ⁢                                                                  ⁢                                  2                  /                  R                                ⁢                                                                  ⁢                1                                            1                +                                  j                  ⁢                                      w                                          w                      0                                                                                            ⁢                          (                                                w                  0                                =                                  1                                      R                    ⁢                                                                                  ⁢                    2                    ⁢                    C                    ⁢                                                                                  ⁢                    2                                                              )                                                          Formula        ⁢                                  ⁢        1            
A formula for the phase of the low pass filter is as follows:
                              tan          ⁢                                          ⁢          ϕ                =                              w                          w              0                                ⁢                      (            phase            )                                              Formula        ⁢                                  ⁢        2            
A formula for the complex filter shown in FIG. 5b is as follows:
                                                                        Vout                Vin                            =                            ⁢                                                R                  ⁢                                                                          ⁢                                      2                    /                    R                                    ⁢                                                                          ⁢                  1                                                  1                  +                                      sR                    ⁢                                                                                  ⁢                    2                    ⁢                    C                    ⁢                                                                                  ⁢                    2                                    -                                      j                    ⁢                                                                                  ⁢                    R                    ⁢                                                                                  ⁢                                          2                      /                      Rx                                                                                                                                              =                            ⁢                                                                    R                    ⁢                                                                                  ⁢                                          2                      /                      R                                        ⁢                                                                                  ⁢                    1                                                        1                    +                                          j                      ⁡                                              (                                                                              w                                                          w                              0                                                                                -                                                                                    R                              ⁢                                                                                                                          ⁢                              2                                                        Rx                                                                          )                                                                                            ⁢                                  (                                                            w                      0                                        =                                          1                                              R                        ⁢                                                                                                  ⁢                        2                        ⁢                        C                        ⁢                                                                                                  ⁢                        2                                                                              )                                                                                        Formula        ⁢                                  ⁢        3            
A formula for the phase of the complex filter is as follows:
                              tan          ⁢                                          ⁢          ϕ                =                              w                          w              0                                -                                                    R                ⁢                                                                  ⁢                2                            Rx                        ⁢                          (              phase              )                                                          Formula        ⁢                                  ⁢        4            
Therefore, the output Vout occurs phase variation of R2/Rx.